The concept of a moveable gate Field Effect Transistor (FET) has been extensively studied and reported in literature. A number of devices have been disclosed that make use of a moveable gate FET for building accelerometers.
Force balance feedback control of vibration sensors has been used in seismometers and in accelerometers for attaining increased bandwidth and dynamic range. A number of devices using this approach have been routinely demonstrated and its theory of operation is well understood. Several seismometer and accelerometer manufacturers base their designs on this principle.
The fabrication of a silicon accelerometer using wafer bonding techniques is disclosed in great detail in U.S. Pat. No. 5,095,752 and No. 5,417,312. In these invention disclosures a relatively large mass made of silicon is encapsulated in a cavity formed by electrodes made out of glass on silicon. The accelerometer is operated using an active feedback loop, in which control voltages are applied to the upper and lower electrodes. The displacement of the free mass under acceleration requires compensation through changes in the voltage applied between the upper and lower electrodes and the moving mass.
Utilizing the FET concept, the feedback bulk silicon micromachined accelerometer disclosed in U.S. Pat. No. 5,205,171 makes use of a feedback loop and at least one pair of dual electrodes acting as capacitive transducers. Differential sensing of the beam-gate capacitance variations is used to generate a null feedback signal used to modulate the voltage applied to electrodes in order to prevent them from moving. Based on the similar concept of using a FET, a device with acceleration dependent gain is disclosed in U.S. Pat. No. 5,103,279 and a device that uses a piezoelectric device to generate voltage for the gate is disclosed in U.S. Pat. No. 4,873,871.
The type of accelerometers mentioned above have good sensitivity at low frequencies but limited sensitivity at high frequencies, although they result in higher bandwidth devices than open loop devices. In addition, due to mechanical and electronic manufacturing variations from one accelerometer to the next, the process of calibrating any given accelerometer to detect vibration as differentiated against a "zero" vibration level is difficult without means to establish such a differential reading.
One fundamental problem of implementing a vibration sensor is that most of the time the fabrication process used to implement the sensor is incompatible with the most common processes used to implement the standard electronics associated with the sensor. To solve this incompatibility, it is often preferred to fabricate the sensor in a separate die from the electronic circuitry. This type of multi-die implementation, however, results in higher costs and lower yield since a more complex multi-die packaging is needed. In some cases cost considerations might advise the use of a monolithic (single-dye) implementation in which the sensor and accompanying electronics reside in the same die.